We report the design, fabrication, photonic packaging and the characterization of a silicon polarization independent optical tunable filter circuit with fiber assembly. We demonstrate the polarization transparent filter characteristics with an insertion loss of ~13.4 dB, an extinction ratio of ~20 dB, and a 3dB bandwidth of 0.2 nm. The tuning range is of ~11.72 nm, along with the tuning speed of less than 400 μs. The tuning efficiency is ~0.23 nm/mW. The use of polarization diversity scheme and the silicon photonic packaging bridges the gap between the silicon photonic circuits and the real applications.
© 2011 OSA
Recently, silicon photonics has exhibited great potential as a promising platform for compact, low-cost photonic integrated circuit (PIC) [1–3] to satisfy various applications’ requirement, such as optical telecommunications, optical interconnects for high speed computers, signal processing as for RF-/microwave-photonics and emerging applications in sensors.
Among numerous functionalities, tunable optical filters [2,3] have important application in fiber optic communications and other optical fields. There are different ways to achieve the tuning ability including the electro-optical, thermo-optical, and piezoelectric effect, etc. Compared with the optical tunable filter in other material platform, CMOS-compatible silicon tunable filter can offer the advantages of compact size, low-cost, low power consumption, and integrate-ability. However, for practical application in optical transmission system, polarization diversity in the PIC is normally required. The large structural birefringence of the silicon waveguide will cause polarization mode dispersion, polarization dependent loss, and polarization dependent wavelength characteristics. All these factors become the obstacle for the application of silicon photonics devices. The most common way to realize polarization independent PIC is to implement polarization diversity scheme [4,5]. The TE- and TM-polarized components of the input light will be split into two paths and then converted to single polarization so that the polarization dependent structures in the PIC will have the identical performance for both paths. This can be realized by using fiber-to-waveguide grating coupler  or the combination of the polarization splitter [5–8] and rotator [5,8–12].
In order to realize practical applications of silicon photonics device, the PIC must be packaged to couple to the single-mode fiber with high efficiency. Different techniques have been investigated in emerging research activities on silicon photonics packaging . Among these, laser welding packaging has the potential to be a promising method as it offers better strength, cleanliness, and long-term reliability . However, the silicon photonic packaging using laser welding technique and fully packaged silicon photonics devices have been rarely reported.
Here we report a fully packaged polarization independent microring resonator-based circuit with fiber assembly for the application of an optical tunable filter. By employing the polarization diversity scheme, a polarization transparent optical tunable filter was demonstrated. We also investigate the photonic packaging process of the silicon photonics device with fiber assembly, which could be easily adopted for other silicon PIC to make practical devices. Using the thermo-optical effect in silicon, the packaged device can be linearly tuned with uniform insertion loss, high extinction ratio and low power consumption.
2. Device Designs and Principle
Figure 1 (a) shows the schematics of the silicon polarization independent optical tunable filter based on microring resonator. The microring resonator is acting as an add-drop filter here, only the wavelengths at resonance will be dropped through the drop port of the filter and then output to the fiber. In order to achieve the tuning ability, a micro-heater is placed on top of the microring resonator. By applying the electrical signal on the heater, the microring resonator will be heated up. Through the thermo-optical effect, the refractive index of the silicon waveguides will be changed, which will result in the resonance red-shift, thus realize the filter tuning function. Due to the asymmetric silicon waveguide structure, the microring resonator has different wavelength response for TE and TM modes. Therefore, to make the device functional, only single polarization state is allowed to pass through the microring resonator.
In the optical tunable filter circuit, the input lightwave from fiber is first split to the TE and TM components propagating in two separate waveguides respectively by the polarization splitter. In our design the mode-coupling based polarization splitter was adopted . The structure is a directional coupler. When proper coupling length is selected, the TM mode will couple into the neighboring waveguide, while the TE mode will remain in the same waveguide. After the splitter, the TE part of the light propagates to the microring resonator. The TM part of the light is rotated 90° to TE mode by a polarization converter (shown in Fig. 1), which comprises of a two-layer polarization rotator  and a mode converter . The whole polarization converter functions for both TE and TM mode in bi-direction. Compared with NTT’s polarization rotator based on an off-axis double-core structure which employed electron beam lithography to meet the tight fabrication tolerance requirement , our design demonstrated larger fabrication tolerance, wider wavelength operation window and better polarization extinction ratio of 15 dB .
After the polarization conversion, a single polarization for both paths is achieved at the microring resonator. The dropped wavelengths at resonance will pass the microring resonator to the right-hand waveguides. Similarly, in the upper path after the microring resonator, the TE signal is first rotated to TM polarization by another polarization converter and then combined with the TE signal from the other path at another polarization splitter. The combined lightwave with both TE and TM components is sent to the output fiber.
3. Device Fabrication
The polarization independent circuit was fabricated on an 8-inch 400-nm-thick SOI wafer with 2-μm-thick buried oxide layer using CMOS-compatible technology. The device layout was defined by 248-nm Deep UV photolithography and transferred onto the device layer by dry etching. The 2-μm-thick top oxide cladding layer was deposited by High Density Plasma PECVD (HDP SiO2).
Figure 2(a) shows the top-view optical micrograph of the fabricated chip. The total length of the chip is 3 mm. The waveguides are tapered to ~180 nm width (limited by 248-nm Deep UV photolithography) to form the invert tapered mode size inverters  (as shown in Fig. 2(b)) at both the input and output ends in order to achieve efficient coupling with lensed fiber. The best efficiency can be achieved with the lensed fiber of 2-2.5 μm focused spot size. Inset (i) shows the directional coupler as a polarization splitter. The waveguide dimension is ~400 nm wide by 200 nm high and the gap separation between waveguides is ~480 nm as shown in Fig. 2(c). The coupling length is ~10 μm. Inset (ii) shows the zoom-in view optical micrograph of the fabricated microring resonator. The racetrack microring arc radius is 7.5 μm, and the straight interaction length is 1 μm. The waveguides are 500 nm wide. The fabricated gap separation between the microring and the straight waveguides is about 200 nm. Figure 2(d) shows the transition region of the polarization rotator. The two-layer structure can be clearly observed. The length of the main rotation region is 30 μm. In order to reduce the power consumption of electrodes, the micro-heater was made with high resistivity material TiN of 120 nm thickness and 1 μm width and the low resistivity material aluminum was used as the electrodes as shown in Fig. 2(e). Since the microring resonator waveguide is 200 nm high, after the HDP SiO2 deposition, the micro-heater deposition was not affected much by the slightly non-uniform surface which can be observed from the SEM as shown in Fig. 2(e). However, for further optimization of the fabrication process, we could consider the CMP process to flatten the surface before the TiN deposition.
4. Photonic Packaging with Fiber Assembly
After fabricating the silicon photonics chips, we employed the well known laser welding technique for the photonic packaging of the chips with fiber assembly. Figure 3(a) depicts the schematic of the packaging structure. We remark that in order to ease the packaging alignment tolerance, as a trade-off, the focused spot diameter of the lensed fiber is designed to be about 3.8 μm instead of the ideal case of 2-2.5 μm, which will sacrifice the coupling efficiency a little. Figure 3(a) inset shows the top-view optical micrograph of the fiber-to-chip coupling.
At first, the wire bonding was processed to connect the metal pads of the micro-heater with the pins on the PCB so that the input electrical signal can be applied via electric wires. Then the first lensed fiber surrounded by a nickel based metal ferrule was passively aligned using a precision vision system and then welded with nickel based weld clips by YAG lasers. The laser welding sequences follows the procedure as detailed in . A total of four welds were placed in pairs at the same height as the center-line of the metal ferrule to minimize Post-Weld-Shift (PWS). The pre-welding offset  between two pairs of welds was 2.0 µm. The power of YAG laser for joining was 9 J. Before this joining, the weld clip was welded on Kovar plate by the YAG laser with power of 12-15 J.
During the welding process, the PWS between the weld clip and the ferrule caused by rapid solidification of the welded region and the associated material shrinkage resulted in the misalignment between the lensed fiber and the circuit. In order to compensate this misalignment, we implemented mechanical tuning via monitoring the waveguide output by a high intensity Infra-Red (IR) camera. The mechanical tuning of the lensed fiber was performed using seesaw effects  and the jointed weld clip acted as a pivot when the rear side of metal ferrule surrounding the lensed fiber was tuned. The second lensed fiber attached by laser welding was then carried out with active alignment. Similarly, misalignment compensation monitored by IR camera was done. Figure 3(b) shows the packaged sub-assembly using YAG laser welding. At last, housing process was conducted by loading the sub-assembly in a designed metal box and then they were assembled using thermal epoxy as shown in Fig. 3(c). The thermal epoxy curing temperature and time were 90°C and 2 hours, respectively.
5. Characterization of the Packaged Device
Figure 4 shows the measured spectra of the packaged silicon optical tunable filter upon various applied voltage. A broadband ASE source and an optical spectrum analyzer (OSA: ANDO AQ6317B) were used to characterize the device. The resolution for OSA measurement is set to be 0.01 nm. As we know, the microring resonator is a polarization dependent structure, which normally shows different resonance characteristics for different polarization with the asymmetric waveguide dimension. For our packaged optical tunable filter using polarization diversity scheme, the measured spectra only exhibit one set of resonances, regardless of the input polarization status.
The spectra have uniform peak transmittances and extinction ratios during the tuning operation. The polarization dependent loss was measured to be less than 0.5dB at the peak wavelengths . The extinction ratio of the filter is about 20 dB. The total insertion loss of the packaged silicon optical tunable filter is 13.4 dB at resonance “*” of 1543.94 nm. We attribute the major contribution of the insertion loss to the fiber-to-chip coupling loss, the polarization splitter induced loss, the polarization converter insertion loss and the propagation loss. As we mentioned above the larger spot-sized lensed fibers introduce larger coupling loss when coupling with the invert taper mode size converter. The misalignment during the packaging process will also introduce the extra coupling loss. The polarization splitter induced loss is coming from the extra loss of the excess TM component in the circuit that results from the down grade polarization extinction ratio due to the fabrication error along the coupling region (e.g. the fabrication induced gap separation variation). The polarization converter loss is mainly due to the fabrication induced sidewall roughness and the tip structure fabrication error as mention in .
The measured FSR of the microring resonator filter is ~11.72 nm. The 3dB bandwidth is about 0.2 nm. While the 10dB bandwidth is about 0.5 nm and the 20dB bandwidth is about 1.8 nm. Upon 3.2 V applied voltage, the resonance “*” of the filter (1543.94 nm at zero voltage) will be tuned 10.84 nm of red-shift. Even larger voltage will result in the resonance shift falling into the adjacent FSR. From above, we note that the tuning range of the packaged silicon optical tunable filter can cover the whole FSR of ~11.72 nm.
Figure 5 shows the output wavelength of the optical tunable filter for different resonance as a function of the power consumption upon applied voltage. The resistance of the packaged device is about 220 Ω. We observed that the wavelength shifting in different FSR almost follows linear trend with the power consumption. The linear fitting suggests a tuning efficiency of ~0.23 nm/mW. The maximum power consumption to cover the whole tuning range for the tunable filter is ~50 mW. We note that although in general the periodic resonance response of the microring resonator filter limits the tuning range to a single FSR, the operation range of the device can actually cover several FSR. We have tested the packaged optical tunable filter in an optical communications system by using its switching ability. The purpose of the device is to switch the optical signal between channel 24 (1558.17 nm based on ITU grid) and channel 30 (1553.33 nm) occasionally. The channel grids fall in two tuning bands of the optical tunable filter as shown in Fig. 5. The tuning voltage for channel 24 and 30 was ~1.5 V (power consumption of ~10.6 mW) and ~3.0 V (~41.2 mW) respectively. By switching the electrical signal between these two voltages, the channel switching was attained. For such single wavelength channel switching applications, the microring resonator-based optical tunable filter is capable for larger range tuning than just within a FSR.
Figure 6 shows the estimated temperature change ΔT as a function of electrical power consumption. For the microring resonator, the wavelength shift results from the refractive index change induced by the thermo-optical effect in silicon. By knowing the wavelength shift Δλ, the silicon effective refractive index change Δneff when wavelength is tuning can be calculated by Δneff = neff(λ') - neff(λ) = Δλ / λ × neff(λ) based on the wavefront phase matching condition, where λ is the original wavelength, λ' is the wavelength after tuning, neff(λ) is the wavelength-dependent effective refractive index of silicon nanowaveguide. The Δneff includes the dispersion induced change (negative for wavelength red-shift) and the silicon refractive index induced change caused by the thermo-optical effect (positive). We remark that here the dispersion in silicon waveguides is actually hurting the tuning efficiency. Without the dispersion, it would take much less temperature change to achieve the same wavelength tuning. By employing the commercial-available full-vector beam propagation method (BPM) optical simulation tool , we can derive the refractive index change Δn of silicon based on the value of neff(λ') at each wavelength and the waveguide structure using iteration method. Hence, the temperature change can be estimated by ΔT = Δn/(δn/δT), where the thermal-induced change in the refractive index of silicon δn/δT is 1.86×10−4 K−1 (at 1550 nm). We observed an almost linear response between the power consumption and the temperature change within the tuning range. The ΔT rate is ~13.2 °C/nm. From the relation between the wavelength shift and the temperature change of the microring resonator-based optical tunable filter, we note that the tuning range of the microring resonator-based thermo-optical tunable filter will be not only limited by the microring resonator dimensions but also by the heater material and the thermo stability. For example, 20-nm wavelength shift may results from a ΔT of ~264 °C, which might not be desirable for the long term stable operation in the real applications.
The tuning speed of the optical tunable filter was also characterized by applying a 2 MHz square-wave electrical driving signal on the micro-heater. Figure 7 shows the driving electrical waveform with 0 – 3 V signal levels and the resulting optical waveform at 2 MHz modulation frequency (at a probe wavelength of ~1553.3 nm). We observed that the rise time is 95 μs, while the fall time is about 1 μs. The longer rise time suggests that the heat up process is slower than the heat dissipation. However, for practical usage, the tuning speed required will be much longer than the transient response time. It will take longer time for the device to reach the thermo equilibrium state in order to achieve stable output. From the measurement, we could estimate the tuning speed of less than 400 μs in a conservative way.
A packaged polarization independent silicon optical tunable filter with fiber assembly was reported. Table 1 summarized the optical specifications of the packaged silicon optical tunable filter. We remark that the relatively high insertion loss and broad bandwidth @ −20dB deserve the further optimization of the filter design and performance. The packaged silicon photonics tunable filter is suitable to the application in wavelength tunable WDM network. With the polarization diversity scheme and the photonic packaging, silicon PIC can be connected with fiber optics and will have more real applications.
This work was supported by the Science and Engineering Research Council of A*STAR (Agency for Science, Technology and Research), Singapore. The SERC grant number: 0921150116.
References and links
1. Y. A. Vlasov, F. Xia, S. Assefa, and W. M. J. Green, “Silicon micro-resonators for on-chip optical networks,” Proceeding of CLEO/QELS, 1 – 2 (2008).
2. Y. Ding, M. Pu, L. Liu, J. Xu, C. Peucheret, X. Zhang, D. Huang, and H. Ou, “Bandwidth and wavelength-tunable optical bandpass filter based on silicon microring-MZI structure,” Opt. Express 19(7), 6462–6470 (2011). [CrossRef]
3. M. A. Popović, T. Barwicz, M. S. Dahlem, F. Gan, C. W. Holzwarth, P. T. Rakich, H. I. Smith, E. P. Ippen, and F. X. Kärtner, “Tunable, fourth-order silicon microring-resonator add-drop filters,” in the 33th European Conference on Optical Communication, ECOC (2007).
4. W. Bogaerts, D. Taillaert, P. Dumon, D. Van Thourhout, R. Baets, and E. Pluk, “A polarization-diversity wavelength duplexer circuit in silicon-on-insulator photonic wires,” Opt. Express 15(4), 1567–1578 (2007). [CrossRef]
5. T. Barwicz, M. R. Watts, M. A. Popović, P. T. Rakich, L. Socci, F. X. Kärtner, E. P. Ippen, and H. I. Smith, “Polarization-transparent microphotonic devices in the strong confinement limit,” Nat. Photonics 1(1), 57–60 (2007). [CrossRef]
7. H. Fukuda, K. Yamada, T. Tsuchizawa, T. Watanabe, H. Shinojima, and S. Itabashi, “Ultrasmall polarization splitter based on silicon wire waveguides,” Opt. Express 14(25), 12401–12408 (2006). [CrossRef]
11. J. Zhang, M. Yu, G. Lo, and D. L. Kwong, “Silicon waveguide based mode-evolution polarization rotator,” IEEE J. Sel. Top. Quantum Electron. 16(1), 53–60 (2010). [CrossRef]
13. L. Zimmermann, G. Battista Preve, T. Tekin, T. Rosin, and K. Landles, “Packaging and assembly for integrated photonics–a review of the ePIXpack photonics packaging platform,” IEEE J. Sel. Top. Quantum Electron. 17(3), 645–651 (2011). [CrossRef]
14. Y. Lin, C. Eichele, and F. G. Shi, “Effect of welding sequence on welding-induced-alignment-distortion in packaging of butterfly laser diode modules: simulation and experiment,” J. Lightwave Technol. 23(2), 615–623 (2005). [CrossRef]
16. T. Shoji, T. Tsuchizawa, T. Watanabe, K. Yamada, and H. Morita, “Low loss mode size converter from 0.3μm square Si wire waveguides to singlemode fibers,” Electron. Lett. 38(25), 1669–1679 (2002). [CrossRef]
17. J. H. Song, J. Zhang, H. Zhang, C. Li, and G. Q. Lo, “Si-photonics based passive device packaging and module performance, Opt. Express (submitted to). [PubMed]
18. J. Zhang, H. Zhang, S. Chen, M. Yu, G. Lo, and D. Kwong, “A polarization diversity circuit for silicon photonics,” in Optical Fiber Communication Conference, (2011).
19. BeamPROP, Rsoft Inc., Research Software, http://www.rsoftdesign.com/.